Many analytical systems guide the aqueous analyte of interest through channels or over surfaces made from polymeric material. Most organic analytes will adsorb non-specifically to polymer surfaces upon contact, in particular strongly hydrophobic and/or lipophilic and/or amphiphilic molecules. This may result in very substantial depletion of the analyte solution concentration before reaching the point of analysis, thus yielding grossly incorrect analysis results. The non-specific adsorption is accentuated in microfluidic systems because of the large surface-to-volume ratio. This can lead to significant underestimation of the potency of drugs in detection systems.
One example is microsystems for automated patch clamp systems where the Inhibitory Concentrations (IC50) of a drug panel was recently reported to be >10 times too high for hydrophobic drug molecules due to analyte depletion onto polymer surfaces (Guo, L. et al., “Automated electrophysiology in the preclinical evaluation of drugs for potential QT prolongation”, Journal of Pharmacological and Toxicological Methods 52 (2005), 123-135).
Different methods have been developed to limit the adsorption of analytes including surface coating or adding a surfactant to the analyte solution (Shen, C. et al., “Chemical modification of polysulfone membrane by polyethylene glycol for resisting drug adsorption and self-assembly of hepatocytes”, Journal of Membrane Science (2010); Silvester, S. et al., “Overcoming non-specific adsorption issues for AZD9164 in human urine samples: consideration of bioanalytical and metabolite identification procedures”, Journal of chromatography.B, Analytical technologies in the biomedical and life sciences 893-894 (2012), 134-143.). In many assays the addition of surfactant interferes with the sensitivity and can also be toxic in cellular assays.
Surface coating with polyethylene glycol (PEG) has previously been shown to significantly limit adsorption of proteins like bovine serum albumin (BSA) or fibrinogen. (Bergstrom, K. et al., Reduction of fibrinogen adsorption on PEG-coated polystyrene surfaces, J. Biomed. Mater. Res., 1992, 26, 779-790).
Shen et al., supra, discloses the ability of PEG to limit adsorption of drugs at the μM level from a membrane made of polysulfone-PEG copolymer. However, the use of copolymers is not easily applied to induce low-binding on different polymer materials. Moreover, many drugs have IC50 or Effector Concentrations (EC50) in the nM range instead of the μM range, and the results of Shen et al. were not promising for extending the low-binding properties of hydrophobic drugs into the nM regime.
PEG coatings have previously been made via a photochemical reaction in a multistep procedure where typically first benzophenone in methanol or acetone is spin coated on the surface and thereafter a PEG monoacrylate (PEGA) is added together with UV light (Ulbricht, M. el al., “Photo-induced graft polymerization surface modifications for the preparation of hydrophilic and low-protein-adsorbing ultrafiltration membranes”, Journal of Membrane Science, 115 (1996), 31-47).
Poly(methyl methacrylate) (PMMA) films have been surface-modified by grafting a solution of spin-coated macromonomer, polyethylene glycol monoacrylate (PEGA), in acetone under UV irradiation in ambient air, wherein benzophenone is used as photosensitizer to generate polymer radicals at the surface of the PMMA film (Iguerb, O. et al. “Graft photopolymerization of polyethylene glycol monoacrylate (PEGA) on poly(methyl methacrylate (PMMA) films to prevent BSA adsorption”, Surf. Interface Anal. 2008, 40, 386-390).
The above prior art methods suffer from drawbacks, including the use of low polarity organic solvents which can dissolve or swell many polymer materials, e.g. the widely used polystyrene, and undesired side reactions since carbon atoms in the organic solvent are also prone to reaction with benzophenone itself. The coating created is typically also rather thick which may be a problem in microfluidic applications. Moreover, the multistep procedure increases the complexity and cost of the coating.
Silicone rubber was coated via photochemical immobilization technology to inhibit protein adsorption, cell adhesion, and foreign body reaction to silicone rubber. Coating reagents were synthesized with 4-benzoylbenzoic acid (BBA) as the photoreactive moiety coupled to different polymers such as PEG (polyethylene glycol), mPEG-amine (methoxy PEG-amine), PAAm (polyacrylamide), PVP (polyvinylpyrrolidone), GVGVP (glycine-valine-glycine-valine-proline), CL-GVGVP (GVGVP crosslinked via gamma irradiation), and HA (hyaluronic acid) (DeFife K. M. et al., “Effects of photochemically immobilized polymer coatings on protein adsorption, cell adhesion, and the foreign body reaction to silicone rubber”, Journal of Biomedical Materials Research, vol. 44, no. 3, pp 298-307, 1999).
WO 90/00887 discloses the preparation of polymeric surfaces by covalently bonding polymer molecules to the surface through external activation of latent reactive groups carried by the polymer molecules.
US2009/0226629 A1 discloses a method for fabricating a display substrate, wherein a substrate is coated with an alignment film, whereafter the alignment film is coated with a photoreactive monomer material layer and subsequently an UV light irradiation is performed selectively on the photoreactive monomer material layer in a first region.
WO 2009/091224 A2 relates to a composition for liquid crystal alignment layer, a preparation method of liquid crystal alignment using the same, and an optical film comprising the liquid crystal alignment layer.
WO 2005/040294 A1 relates to an organosilane-based composition for producing a barrier layer for gases, comprising (i) at least one organoalkoxysilane, (ii) at least one aminoalkylalkoxysilane, (iii) at least one polyol, (iv) optionally, another alkoxysilane or alkoxysiloxane, and (v) optionally, at least one nano- or microscale semimetal oxide or metal oxide, semimetal oxide hydroxide or metal oxide hydroxide, or semimetal hydroxide or metal hydroxide and/or (vi) at least one cocondensate, and/or (vii) reaction products produced under hydrolysis conditions, and (viii) an organic solvent.
WO 03/093329 A1 relates to a multi-coating system with gas-barrier properties that is able to undergo crosslinking by means of UV radiation and is said to be particularly suitable for the external protection of containers made of thermoplastic polymers.
DE 101 49 587 B4 relates to a photoreactive coated composite membrane based on a separately prepared matrix membrane, the surface of which is functionalized, wherein a reaction mixture containing at least one functionalising monomer is used to form two different polymer layer structures using two different reaction conditions.
EP 0 274 596 A2 relates to an ultraviolet radiation curable coating composition for plastic substrates comprised of: (i) at least one polyfunctional acrylate monomer; (ii) at least one acetophenone photoinitiator; and (iii) at least one active ultraviolet radiation adsorber selected from benzotriazoles, cyanoacrylates, and hydroxybenzophenones or mixtures thereof.
However, there is still a need for a simple method of coating a polymeric substrate which results in a thin coating and which may be performed in one step without the use of organic solvents in either open or closed volumes (e.g. channel systems), and with the ability to pattern the coated layer on macroscopic and microscopic length scales.